For lapping a cylindrical hole of small diameter it is a common practice to employ a lapping tool constituted by a tube having an expandable working portion which carries diamond cutters externally. Within the tube is provided a cone movable with a sliding motion and integral with a controlling rod. By movement of the cone, the working portion of the lapping tool is expanded.
A lapping tool of this kind normally incorporates, in order to allow the expansion of its working portion, either one longitudinal slot extending from one end of the tube to the other, or a series of alternating longitudinal slots beginning from one end of the tube and terminated at a short distance from the other end. Unfortunately, however, such a lapping tool does not retain a cylindrical form when expanded. It tends to assume a swollen form similar to that of a barrel. This shape prevents lapping with a hole of accurate dimensions. To obtain accuracy within half a micron for instance, it is necessary both that each lapping action remove very little material (otherwise the lapping tool is overworked at the end and is worn to a conical shape) and that the lapping tool retain a perfectly cylindrical shape when expanded.